German Patent 32 48 178, by the inventor hereof, describes an offset printing machine in which, for repeated imaging of a printing forme cylinder, a thermal transfer foil is so activated that hydrophobic substances on a substrate of the foil can be transferred to the forme so that, upon printing, they will accept ink to form inked regions or points on the forme, for subsequent printing on a substrate, or offsetting on an offset cylinder. The transfer foil can be heated by point-heat sources to provide for selective melting of spots or points of the meltable layer on the foil.
Precise transfer of image points of equal size requires that, upon each contact of the heating element of the image recording head with the back side of the substrate of the foil, it transfers the same quantity of heat thereto; further, the mechanical pressure arising in the chain of transfer to the printing forme cylinder must be the same. Even highest accuracy in manufacture of the recording head and all components of the transfer chain cannot eliminate some deviations in spacing and thickness of the transfer foil. Further, roughness of the surface of the forme cylinder causes changes in transfer capability. The changes or variations or tolerances from a design value may be tiny, for example in the region of only a few or several thousands of a millimeter. Yet, variations in printing line contacts or pressures occur, which either cause non-uniform reproducible transfer of image points or particles, with equal energy supply or, in a worst case, make such transfer impossible.